.\" $OpenBSD: SSL_get_client_random.3,v 1.2 2018/03/24 00:55:37 schwarze Exp $
.\" full merge up to: OpenSSL e9b77246 Jan 20 19:58:49 2017 +0100
.\"
.\" This file was written by Nick Mathewson <nickm@torproject.org>
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.Dd $Mdocdate: March 24 2018 $
.Dt SSL_GET_CLIENT_RANDOM 3
.Os
.Sh NAME
.Nm SSL_get_client_random ,
.Nm SSL_get_server_random ,
.Nm SSL_SESSION_get_master_key
.Nd get internal TLS handshake random values and master key
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft size_t
.Fo SSL_get_client_random
.Fa "const SSL *ssl"
.Fa "unsigned char *out"
.Fa "size_t outlen"
.Fc
.Ft size_t
.Fo SSL_get_server_random
.Fa "const SSL *ssl"
.Fa "unsigned char *out"
.Fa "size_t outlen"
.Fc
.Ft size_t
.Fo SSL_SESSION_get_master_key
.Fa "const SSL_SESSION *session"
.Fa "unsigned char *out"
.Fa "size_t outlen"
.Fc
.Sh DESCRIPTION
.Fn SSL_get_client_random
extracts the random value that was sent from the client to the server
during the initial TLS handshake.
It copies at most
.Fa outlen
bytes of this value into the buffer
.Fa out .
If
.Fa outlen
is zero, nothing is copied.
.Pp
.Fn SSL_get_server_random
behaves the same, but extracts the random value that was sent
from the server to the client during the initial TLS handshake.
.Pp
.Fn SSL_SESSION_get_master_key
behaves the same, but extracts the master secret used to guarantee the
security of the TLS session.
The security of the TLS session depends on keeping the master key
secret: do not expose it, or any information about it, to anybody.
To calculate another secret value that depends on the master secret,
use
.Xr SSL_export_keying_material 3
instead.
.Pp
All these functions expose internal values from the TLS handshake,
for use in low-level protocols.
Avoid using them unless implementing a feature
that requires access to the internal protocol details.
.Pp
Despite the names of
.Fn SSL_get_client_random
and
.Fn SSL_get_server_random ,
they are not random number generators.
Instead, they return the mostly-random values that were already
generated and used in the TLS protocol.
.Pp
In current versions of the TLS protocols,
the length of client_random and server_random is always
.Dv SSL3_RANDOM_SIZE
bytes.
Support for other
.Fa outlen
arguments is provided for the unlikely event that a future
version or variant of TLS uses some other length.
.Pp
Finally, though the client_random and server_random values are called
.Dq random ,
many TLS implementations generate four bytes of those values
based on their view of the current time.
.Sh RETURN VALUES
If
.Fa outlen
is greater than 0, these functions return the number of bytes
actually copied, which is less than or equal to
.Fa outlen .
If
.Fa outlen
is 0, these functions return the maximum number of bytes they would
copy \(em that is, the length of the underlying field.
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_export_keying_material 3 ,
.Xr SSL_SESSION_get_id 3 ,
.Xr SSL_SESSION_get_time 3 ,
.Xr SSL_SESSION_new 3
.Sh HISTORY
These functions first appeared in OpenSSL 1.1.0
and have been available since
.Ox 6.3 .
